If you want to understand calcareous rocks – you are in the right place!
In this article, you will learn about what a calcareous rock is, some common examples of calcareous rocks, and how you can identify them while rockhounding in nature.
Calcareous Rocks Examples (Characteristics and Types)
What is a Calcareous Rock?
A calcareous rock is composed primarily of calcium carbonate, usually in the form of calcite or aragonite.
The majority of calcareous rocks are found in shallow coastal areas.
This is because a primary source of calcium carbonate is the fossilized remains of the shells and bones of marine organisms.
Limestone is a soft and brittle sedimentary rock with a granular texture that you can easily scratch with your fingernail.
Chemically, limestone is composed of more than fifty percent calcite or aragonite.
These carbonates are frequently mixed with other local sedimentary impurities.
The stratifications we see in limestone are caused by changes in the composition of these impurities in the local sediment as they compounded over time.
Coquina is a very coarse and granular form of limestone.
The large grains are the fossilized remains of marine organisms.
Since these fossilized shells are composed almost entirely of calcite or aragonite, when they form a sedimentary rock it is a variety of limestone.
The distinguishing feature of coquina is that it is extremely porous with large grains and without contamination from clays and silts.
The grains adhere together when calcium carbonate precipitates in a process called cementation.
The result is a roughly porous white stone that is easily distinguished from nonporous limestone.
Coquina is most frequently found in tropical or subtropical waters, where most marine fossils are produced.
Marble is a harder, crystallized form of limestone.
In nature, you can find it as a stone that is white or blue-gray, yellow, brown, or black.
When limestone is subjected to intense geologic heat and pressure as tectonic plates converge, a process called metamorphism takes place, recrystallizing the calcite or aragonite in the limestone to form a new structure: marble.
Unlike limestone, marble is not stratified.
The impurities in the original limestone create the swirling effect that we think of as “marbling” during the metamorphic process.
Marble is harder than limestone but is still comparably soft and is famously an excellent medium for sculptors.
When sedimentary rock contains a high concentration of dolomite – calcium magnesium carbonate CaMg(CO3)2 – we can call it dolomite, dolomite rock, or dolostone.
The mineral dolomite is quite rare in coastal environments, but the sedimentary rock composed of it (dolostone) is fairly common.
Dolostone is formed when limestone and lime mud are postdepositionally altered by magnesium-rich groundwater which replaces the original calcite with dolomite.
Limestone and dolostone are almost indistinguishable in the field, although dolostone is slightly harder (a 3 1/2 to 4 on the Mohs hardness scale as compared to a 3 for limestone) and is slightly less soluble in dilute hydrochloric acid.
When dolostone is compressed between tectonic plates, it undergoes the same metamorphic process that limestone undergoes to become marble, creating dolomitic marble.
Limestone is made up of sedimentary calcite, but this mineral can also appear in its purest form as beautiful clear or white crystal formations which may also be tinged with blue or pink depending on specific impurities and the region in which they are found.
These crystals are most commonly found in and around limestone or dolostone in shallow coastal areas.
Calcite grows in a rhombohedral formation with straight edges and corners or in six-sided hexagonal crystals.
The chemical composition of calcite is identical to aragonite – calcium carbonate – but in a different molecular configuration.
Sometimes calcite is confused with quartz, which looks similar but has an entirely different silica-based composition.
You can tell the difference by examining the faces of the crystals.
Quartz has no cleavage on the hexagonal prisms or basal faces, while calcite has perfect or very good cleavage.
Aragonite is another calcium carbonate crystal that is slightly less common than calcite.
Pure aragonite is white although the coloration can vary due to the presence of other minerals and according to region.
Chemically, aragonite and calcite are both calcium carbonate in different molecular configurations.
Calcite is the more stable configuration and over long periods aragonite will degrade into calcite.
Aragonite forms four-sided orthorhombic crystals, distinguishing them from the six-sided hexagonal or rhombohedral crystals in calcite.
It can be difficult to tell these two varieties apart because sometimes aragonite crystals form in twin structures that join together into a pseudohexagonal crystal called a sixling, which resembles calcite.
Onyx or Banded Calcite
Don’t confuse this with the gem onyx, which is technically black and white parallel banded agate- in this context, what we call onyx is banded calcite.
You can find this smooth, nonporous stone in a variety of colors from brown and red to pink and blue. The colors are “banded” in stratification.
Onyx, or banded calcite, is formed by calcium carbonate-rich springwater that gradually forms a crust of calcite crystals at the mouth or opening of a spring.
As the water rushes over the crystals, depositing more calcite while also eroding the crystals, it grows into banded calcite.
The “bands” or different colors are created by differences in the rate of flow of the spring, as well as the changing mineral content.
Travertine is, like banded calcite, composed of calcite and stratified. It is typically white or yellow, but it can be red or orange in iron-rich environments.
Travertine forms in geothermal hot springs or caves in the form of stalactites and stalagmites.
What differentiates travertine from banded calcite (both are composed entirely of calcite) is that banded calcite is smooth and nonporous, while travertine is porous.
The pores in travertine are created by organisms that live in these environments: bryophytes, macrophytes, and algae.
What We Learned
Calcareous rocks are each made up of some form of calcium carbonate (calcite, aragonite, or dolomite).
However, they are very different in their molecular composition, the processes that create them, and their physical properties.
Sedimentary limestone, metamorphic marble, and travertine stalactites are all calcareous rocks.
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